1. Field of the Invention
The invention relates to a valve characteristic control device for an internal combustion engine including a variable valve mechanism that varies a lift time area of an intake valve, and a method of the same.
2. Description of the Related Art
Internal combustion engines including a variable valve mechanism that varies a valve characteristic of an intake valve, etc., are in practical use. Various types of variable valve mechanism are known, such as a variable valve timing mechanism that varies valve timing, a variable lift mechanism that varies a valve lift amount, and a mechanism described in Japanese Patent Application Publication No. JP 2001-263015 A that varies an opening period and a lift amount of a valve.
More specifically, with a variable valve mechanism that can continuously vary a valve lift amount and an opening timing of an intake valve, as described in Japanese Patent Application Publication No. JP 2001-263015 A, drive control of the variable valve mechanism can be performed to adjust an intake air amount. The intake air amount that changes along with variation of the valve characteristics has a correlation with an integral of the valve lift amount during a time period from when the intake valve is opened to when it is closed. Hereinafter, this type of time integral will be referred to as “lift time area”. The variable valve mechanism as described in Japanese Patent Application Publication No. JP 2001-263015 A is able to vary a valve characteristic related to this lift time area, and thus vary a valve characteristic that is related to adjustment of the intake air amount.
Drive control of the above mechanism, namely, the variable valve mechanism that varies the valve characteristic related to the lift time area of the intake valve, may be performed to adjust the intake air amount of the internal combustion engine. In this case, drive of the variable valve mechanism is controlled in a feedback manner in order to make a present value of the valve characteristic become equal to a target value that is calculated based on an engine operating state. More particularly, a difference between the target value of the valve characteristic and the present value is used as a basis for performing feedback control of a drive amount of an actuator etc. that drives the variable valve mechanism. In this type of feedback control system, a control gain is set in order to achieve appropriate responsiveness and stability. The control gain is a value that adjusts the drive amount of the actuator etc. in accordance with the difference. As the value of the control gain increases, the drive amount increases, thereby increasing the responsiveness of the feedback control system. On the other hand, as the value of the control gain decreases, the drive amount also decreases, thereby increasing the stability of the feedback control system.
When the intake air amount of the engine is small, a ratio of the change of the intake air amount with respect to a change amount of the value characteristic increases. On the other hand, when the intake air amount is large, the change ratio has a tendency to decrease. This ratio of the change of the intake air amount with respect to the change amount of the valve characteristic will be referred to hereinafter as “air sensitivity”. When the ratio of the change of the intake air amount with respect to the change amount of the valve characteristic is large, the air sensitivity is said hereinafter to be “high”. On the other hand, when the ratio of the change of the intake air amount with respect to the change amount of the valve characteristic is small, the air sensitivity is said hereinafter to be “low”.
The air sensitivity changes in accordance with the intake air amount as described above. Accordingly, if the control gain is set to a constant value, it is possible that the following problems will occur. When the air sensitivity is in a low region (a region where it is difficult for the air intake amount to change), the drive amount of the variable valve mechanism with respect to the change amount of the required intake air has a tendency to become insufficient. As a result, there is a possibility that responsiveness of the feedback control system will deteriorate and that problems will occur like, for example, change of the intake air amount becoming slow. On the other hand, when the air sensitivity is in a high region (a region where it is easy for the intake air amount to change), the drive amount of the variable valve mechanism with respect to the change amount of the required intake air becomes excessive. Accordingly, there is a possibility that stability of the feedback control system will deteriorate and that problems will occur like, for example, overshoot of the intake air amount causing the engine to stall.